Split items/ and trainers/ into their data/ and engine/ components

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/golduck.asm

@@ -0,0 +1,10 @@
+; GolduckPokedexEntry: ; 0x182d55
+	db "DUCK@" ; species name
+	dw 507, 1690 ; height, weight
+
+	db   "It swims grace-"
+	next "fully along on the"
+	next "quiet, slow-moving"
+	page "rivers and lakes"
+	next "of which it is so"
+	next "fond.@"